[time-nuts] Re: Optimizing GPSDO for phase stability

[Bob kb8tq]

Hi

What is the “customer” after? 

For a PPS, it could be the offset from UTC. This gets into GPS to UTC and then into
GPS master to local GPS pulse. Bottom line usually is that the raw GPS pulse is 
the answer to “close to UTC”. Equally it could be a PPS used for metrology ( = ADEV 
measurement). Then you want the lowest ADEV PPS. The answer here is a PPS divided
off the local oscillator with nothing else done to it. 

Different customers, different needs, different answers. The same applies to how
the local oscillator is disciplined. 

What to do (without building a dozen different designs?) ….. give the customer a 
software setting that lets them pick what they get. Write up a couple dozen pages
on why you would use one or the other. Yes, the software switch takes about two 
minutes to code. The pages of “yack” may take a couple weeks to fully sort out. 

Bob

> On May 27, 2022, at 8:02 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> The GPSDO/Timer/Counter I'm building also is intended to have a stabilized PPS output (so with GPS jitter removed).
> The output PPS is created by multiplying/dividing the 10MHz of a disciplined TCXO up and down to 1 Hz using a PLL and a divide by 2e8. No SW or re-timing involved.
> The 1 PPS output is phase synchronized with the PPS using a SW control loop and thus should be a good basis for experiments that require a time pulse that is stable and GPS time correct.
> As I have no clue how to specify or evaluate the performance of such a PPS output I've done some experiments.
> In the first attached graph you can see the ADEV of the GPS PPS (PPS - Rb) and the 1 PPS output with three different control parameters (Tick - RB)
> As I found it difficult to understand what the ADEV plot in practice means for the output phase stability I also added the Time Deviation plot as I'm assuming this gives information on the phase error versus the time scale of observation.
> Lastly a plot is added showing the Phase Difference. All plots where created using the linear residue as the Rb used as reference is a bit out of tune.
> Also the TIM files are attached
> The "PPS - RB" and "Tick - RB Kp=0.04" where measured simultaneously and should show the extend to which the GPS PPS is actually drifting in phase versus the Rb and how this impacts the output phase of the stabilized output PPS.
> My conclusion is that a higher then expected Kp of 0.1 gives the most stable output phase performance where the best frequency performance is realized with a Kp = 0.04
> I welcome feedback on the interpretation of these measurements and the application of output phase stabilization.
> <Allan_deviation.png><Time_Deviation.png><phase_difference.png><PPS - Rb.tim><Tick - Rb Kp=0.1.tim><Tick - Rb Kp=0.01.tim><Tick - Rb Kp=0.04.tim>_______________________________________________
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